This invention relates to power supply systems, and more specifically to those of the switching type for powering both a primary and a secondary load, the secondary load being significantly less in power requirement than the first. The switching power supply system according to the invention is particularly well suited for use in or with remotely controllable television receivers, among other applications.
A greater majority of television sets today are remotely controllable. Such TV sets have an inbuilt remote control receiver circuit for receiving signals from the remote control unit. The remote control receiver circuit must be powered even when the main electric circuitry of the TV set is not. The power supply systems of remotely controllable TV sets are therefore required to possess a circuit for powering the main circuitry (primary load) and another for powering the remote control receiver circuit (secondary load). The provision of two totally independent power supply circuits would be undesirable, however, because the resulting system would be too bulky and costly.
A solution to this problem is found for example in Japanese Unexamined Patent Publication No. 4-308465, in which the two power supply circuits are combined so as to share some components. More specifically, this prior art power supply system comprises a switching transistor for on-off control of a unidirectional supply voltage, and a transformer having a primary winding connected in series with the switching element, a secondary winding for powering the main TV set circuitry, and a tertiary winding for powering the remote control receiver circuit. The switching transistor is controlled so as to make constant the supply voltage applied to the main TV set circuitry.
Customarily, the switching frequency of such TV set power supply systems has been as high as from 40 to 100 kilohertz, with a view to the reduction of the size of the transformer and the suppression of noise in the audio frequency range. However, the constant switching of the direct current power supply at such high frequencies inevitably involves considerable power losses. The trouble with the prior art switching power supply system is that such high switching frequencies have been maintained not only when the main TV set circuitry is being powered but also when it is not, that is, when only the remote control receiver circuit is. Naturally, TV sets are held switched on for a much shorter period of time than off.